Postfix TLS Support

WARNING

By turning on TLS support in Postfix, you not only get the
ability to encrypt mail and to authenticate remote SMTP clients or servers.
You also turn on thousands and thousands of lines of OpenSSL library
code. Assuming that OpenSSL is written as carefully as Wietse's
own code, every 1000 lines introduce one additional bug into
Postfix.

What Postfix TLS support does for you

Transport Layer Security (TLS, formerly called SSL) provides
certificate-based authentication and encrypted sessions. An
encrypted session protects the information that is transmitted with
SMTP mail or with SASL authentication.

NOTE: This document describes a TLS
user interface that was introduced with Postfix version 2.3. Support
for an older user interface is documented in TLS_LEGACY_README,
which also describes the differences between Postfix and the
third-party patch on which Postfix version 2.2 TLS support was
based.

In order to use TLS, the Postfix SMTP server generally needs
a certificate and a private key. Both must be in "PEM" format. The
private key must not be encrypted, meaning: the key must be accessible
without a password. The certificate and private key may be in the same
file, in which case the certificate file should be owned by "root" and
not be readable by any other user. If the key is stored separately,
this access restriction applies to the key file only, and the
certificate file may be "world-readable".

Public Internet MX hosts without certificates signed by a
well-known public CA must still generate, and be prepared to present
to most clients, a self-signed or private-CA signed certificate.
The remote SMTP client will generally not be able to verify the
self-signed certificate, but unless the client is running Postfix
or similar software, it will only negotiate TLS ciphersuites that
require a server certificate.

For servers that are not public Internet MX hosts, Postfix
supports configurations with no certificates. This entails the
use of just the anonymous TLS ciphers, which are not supported by
typical SMTP clients. Since such clients will not, as a rule, fall
back to plain text after a TLS handshake failure, a certificate-less
Postfix SMTP server will
be unable to receive email from most TLS enabled clients. To avoid
accidental configurations with no certificates, Postfix enables
certificate-less operation only when the administrator explicitly sets
"smtpd_tls_cert_file = none". This ensures that new Postfix
SMTP server configurations will not accidentally run with no
certificates.

RSA, DSA and ECDSA (Postfix ≥ 2.6) certificates are supported.
Most sites only have RSA certificates. You can configure all three
at the same time, in which case the ciphersuite negotiated with the
remote SMTP client determines which certificate is used. If your
DNS zone is signed, and you want to publish RFC 6698 TLSA records,
these must match any of the configured certificates. Since the
best practice is to publish "3 1 1" certificate associations, create
a separate TLSA record for each public-key certificate digest.

Creating the server certificate file

To verify the Postfix SMTP server certificate, the remote SMTP
client must receive the issuing CA certificates via the TLS handshake
or via public-key infrastructure. This means that the Postfix server
public-key certificate file must include the server certificate
first, then the issuing CA(s) (bottom-up order). The Postfix SMTP
server certificate must be usable as SSL server certificate and
hence pass the "openssl verify -purpose sslserver ..." test.

The examples that follow show how to create a server certificate
file. We assume that the certificate for "server.example.com" was
issued by "intermediate CA" which itself has a certificate issued
by "root CA".

With legacy public CA trust verification, you can omit the
root certificate from the "server.pem" certificate file. If the
client trusts the root CA, it will already have a local copy of the
root CA certificate. Omitting the root CA certificate reduces the
size of the server TLS handshake.

% cat server_cert.pem intermediate_CA.pem > server.pem

If you publish RFC 6698 TLSA "2 0 1" or "2 1 1" records to
specify root CA certificate digests, you must include the corresponding
root CA certificates in the "server.pem" certificate file. See the
documentation of the tls_dane_trust_anchor_digest_enablemain.cf
parameter.

% cat server_cert.pem intermediate_CA.pem root.pem > server.pem

Remote SMTP clients will be able to use the TLSA record you
publish (which only contains the certificate digest) only if they
have access to the corresponding certificate. Failure to verify
certificates per the server's published TLSA records will typically
cause the SMTP client to defer mail delivery. The foregoing also
applies to "2 0 2" and "2 1 2" TLSA records or any other digest of
a CA certificate, but it is expected that SHA256 will be by far the
most common digest for TLSA.

As a best practice, publish either "3 0 1" or "3 1 1" TLSA
associations that specify the SHA256 digest of the server certificate
public key with the alias-expanded hostname of each STARTTLS capable
SMTP server. These continue to work when a certificate is renewed
with the same public/private key pair.

For instructions on how to compute the digest of a certificate
or its public key for use in TLSA records, see the documentation of
the smtpd_tls_fingerprint_digestmain.cf parameter.

When a new key or certificate is generated, an additional TLSA
record with the new digest must be published in advance of the
actual deployment of the new key or certificate on the server. You
must allow sufficient time for any TLSA RRsets with only the old
digest to expire from DNS caches. The safest practice is to wait
until the DNSSEC signature on the previous TLSA RRset expires, and
only then switch the server to use new keys published in the updated
TLSA RRset. Once the new certificate trust chain and private key
are in effect, the DNS should be updated once again to remove the
old digest from the TLSA RRset.

If you want the Postfix SMTP server to accept remote SMTP client
certificates issued by one or more root CAs, append the root
certificate to $smtpd_tls_CAfile or install it in the $smtpd_tls_CApath
directory.

To verify a remote SMTP client certificate, the Postfix SMTP
server needs to trust the certificates of the issuing Certification
Authorities. These certificates in "PEM" format can be stored in a
single $smtpd_tls_CAfile or in multiple files, one CA per file in
the $smtpd_tls_CApath directory. If you use a directory, don't forget
to create the necessary "hash" links with:

# $OPENSSL_HOME/bin/c_rehash /path/to/directory

The $smtpd_tls_CAfile contains the CA certificates of one or
more trusted CAs. The file is opened (with root privileges) before
Postfix enters the optional chroot jail and so need not be accessible
from inside the chroot jail.

Additional trusted CAs can be specified via the $smtpd_tls_CApath
directory, in which case the certificates are read (with $mail_owner
privileges) from the files in the directory when the information
is needed. Thus, the $smtpd_tls_CApath directory needs to be
accessible inside the optional chroot jail.

When you configure the Postfix SMTP server to request client certificates, the DNs of Certification
Authorities in $smtpd_tls_CAfile are sent to the client, in order to allow
it to choose an identity signed by a CA you trust. If no $smtpd_tls_CAfile
is specified, no preferred CA list is sent, and the client is free to
choose an identity signed by any CA. Many clients use a fixed identity
regardless of the preferred CA list and you may be able to reduce TLS
negotiation overhead by installing client CA certificates mostly or
only in $smtpd_tls_CApath. In the latter case you need not specify a
$smtpd_tls_CAfile.

Note, that unless client certificates are used to allow greater
access to TLS authenticated clients, it is best to not ask for
client certificates at all, as in addition to increased overhead
some clients (notably in some cases qmail) are unable to complete
the TLS handshake when client certificates are requested.

To include information about the protocol and cipher used as
well as the client and issuer CommonName into the "Received:"
message header, set the smtpd_tls_received_header variable to true.
The default is no, as the information is not necessarily authentic.
Only information recorded at the final destination is reliable,
since the headers may be changed by intermediate servers.

With this, the Postfix SMTP server announces STARTTLS support to
remote SMTP clients, but does not require that clients use TLS encryption.

Note: when an unprivileged user invokes "sendmail -bs", STARTTLS
is never offered due to insufficient privileges to access the Postfix
SMTP server
private key. This is intended behavior.

You can ENFORCE the use of TLS,
so that the Postfix SMTP server announces STARTTLS and accepts no
mail without TLS encryption, by setting
"smtpd_tls_security_level = encrypt". According to RFC 2487 this
MUST NOT be applied in case
of a publicly-referenced Postfix SMTP server. This option is off
by default and should only seldom be used.

TLS is sometimes used in the non-standard "wrapper" mode where
a server always uses TLS, instead of announcing STARTTLS support
and waiting for remote SMTP clients to request TLS service. Some
clients, namely
Outlook [Express] prefer the "wrapper" mode. This is true for OE
(Win32 < 5.0 and Win32 >=5.0 when run on a port<>25
and OE (5.01 Mac on all ports).

It is strictly discouraged to use this mode from main.cf. If
you want to support this service, enable a special port in master.cf
and specify "-o smtpd_tls_wrappermode=yes" (note: no space around
the "=") as an smtpd(8) command line option. Port 465 (smtps) was
once chosen for this feature.

To receive a remote SMTP client certificate, the Postfix SMTP
server must explicitly ask for one (any contents of $smtpd_tls_CAfile
are also sent to the client as a hint for choosing a certificate from
a suitable CA). Unfortunately, Netscape clients will either complain
if no matching client certificate is available or will offer the user
client a list of certificates to choose from. Additionally some MTAs
(notably some versions of qmail) are unable to complete TLS negotiation
when client certificates are requested, and abort the SMTP session. So
this option is "off" by default. You will however need the certificate
if you want to use certificate based relaying with, for example, the
permit_tls_clientcerts feature. A server that wants client certificates
must first present its own certificate. While Postfix by default
offers anonymous ciphers to remote SMTP clients, these are automatically
suppressed
when the Postfix SMTP server is configured to ask for client
certificates.

The client certificate verification depth is specified with the
main.cfsmtpd_tls_ccert_verifydepth parameter. The default verification
depth is 9 (the OpenSSL default), for compatibility with Postfix
versions before 2.5 where smtpd_tls_ccert_verifydepth was ignored.
When you configure trust in a
root CA, it is not necessary to explicitly trust intermediary CAs signed
by the root CA, unless $smtpd_tls_ccert_verifydepth is less than the
number of CAs in the certificate chain for the clients of interest. With
a verify depth of 1 you can only verify certificates directly signed
by a trusted CA, and all trusted intermediary CAs need to be configured
explicitly. With a verify depth of 2 you can verify clients signed by a
root CA or a direct intermediary CA (so long as the client is correctly
configured to supply its intermediate CA certificate).

Sending AUTH data over an unencrypted channel poses a security
risk. When TLS layer encryption is required
("smtpd_tls_security_level = encrypt"), the Postfix SMTP server will
announce and accept AUTH only after the TLS layer has been activated
with STARTTLS. When TLS layer encryption is optional
("smtpd_tls_security_level = may"), it may however still be useful
to only offer AUTH when TLS is active. To maintain compatibility
with non-TLS clients, the default is to accept AUTH without encryption.
In order to change this behavior, set
"smtpd_tls_auth_only = yes".

The Postfix SMTP server and the remote SMTP client negotiate a
session, which takes some computer time and network bandwidth. SSL
protocol versions other than SSLv2 support resumption of cached
sessions. Not only is this more CPU and bandwidth efficient, it
also reduces latency as only one network round-trip is used to
resume a session while it takes two round-trips to create a session
from scratch.

Since Postfix uses multiple smtpd(8) service processes, an
in-memory cache is not sufficient for session re-use. Clients store
at most one cached session per server and are very unlikely to
repeatedly connect to the same server process. Thus session caching
in the Postfix SMTP server generally requires a shared cache (an
alternative available with Postfix ≥ 2.11 is described below).

To share the session information between multiple
smtpd(8) processes, a session cache database is used. You
can specify any database type that can store objects of several
kbytes and that supports the sequence operator. DBM databases are
not suitable because they can only store small objects. The cache
is maintained by the tlsmgr(8) process, so there is no problem with
concurrent access. Session caching is highly recommended, because
the cost of repeatedly negotiating TLS session keys is high.

Starting with Postfix 2.11, linked with a compatible OpenSSL
library (at least 0.9.8h, preferably 1.0.0 or later) the Postfix
SMTP server supports RFC 5077 TLS session resumption without
server-side state when the remote SMTP client also supports RFC5077. The session is encrypted by the server in a session
ticket returned to client for storage. When a client sends a
valid session ticket, the server decrypts it and resumes the session,
provided neither the ticket nor the session have expired. This
makes it possible to resume cached sessions without allocating space
for a shared database on the server. This feature can be disabled
by setting the session cache timeout to zero, otherwise the timeout
must be at least 2 minutes and at most 100 days.

Note, session tickets can only be negotiated if the client
disables SSLv2 and does not use the legacy SSLv2 compatible HELLO
message. This is true by default with the Postfix ≥ 2.6 SMTP
client.

Note: as of version 2.5, Postfix no longer uses root privileges
when opening this file. The file should now be stored under the
Postfix-owned data_directory. As a migration aid, an attempt to
open the file under a non-Postfix directory is redirected to the
Postfix-owned data_directory, and a warning is logged.

Cached Postfix SMTP server session information expires after
a certain amount of time. Postfix/TLS does not use the OpenSSL
default of 300s, but a longer time of 3600sec (=1 hour). RFC 2246
recommends a maximum of 24 hours.

As of Postfix 2.11 this setting cannot exceed 100 days. If set
≤ 0, session caching is disabled. If set to a positive value
less than 2 minutes, the minimum value of 2 minutes is used instead.

When the Postfix SMTP server does not save TLS sessions to an
external cache database, client-side session caching is unlikely
to be useful. To reduce waste of client resources, the Postfix SMTP server can
be configured to not issue TLS session ids. By default the Postfix
SMTP server always issues TLS session ids. This works around known
interoperability issues with some MUAs, and prevents possible
interoperability issues with other MTAs.

Use the remote
SMTP client certificate fingerprint or public key fingerprint
(Postfix 2.9 and later) as the lookup key for the specified access(5)
table.

The digest algorithm used to compute the client certificate
fingerprints is specified with the main.cfsmtpd_tls_fingerprint_digest
parameter. The default is "md5", for compatibility with Postfix
versions < 2.5.

The permit_tls_all_clientcerts feature must be used with caution,
because it can result in too many access permissions. Use this
feature only if a special CA issues the client certificates, and
only if this CA is listed as trusted CA. If other CAs are trusted,
any owner of a valid client certificate would be authorized.
The permit_tls_all_clientcerts feature can be practical for a
specially created email relay server.

To extract the public key fingerprint from an X.509 certificate,
you need to extract the public key from the certificate and compute
the appropriate digest of its DER (ASN.1) encoding. With OpenSSL
the "-pubkey" option of the "x509" command extracts the public
key always in "PEM" format. We pipe the result to another OpenSSL
command that converts the key to DER and then to the "dgst" command
to compute the fingerprint.

The actual command to transform the key to DER format depends
on the version of OpenSSL used. With OpenSSL 1.0.0 and later, the
"pkey" command supports all key types. With OpenSSL 0.9.8 and
earlier, the key type is always RSA (nobody uses DSA, and EC
keys are not fully supported by 0.9.8), so the "rsa" command is
used.

By default anonymous ciphers are enabled. They are automatically
disabled when remote SMTP client certificates are requested. If
clients are expected to always verify the Postfix SMTP
server certificate you may want to disable anonymous ciphers
by setting "smtpd_tls_mandatory_exclude_ciphers = aNULL" or
"smtpd_tls_exclude_ciphers = aNULL", as appropriate. One can't force
a remote SMTP client to check the server certificate, so excluding
anonymous ciphers is generally unnecessary.

With mandatory and opportunistic TLS encryption, the Postfix
SMTP server by default disables SSLv2 and SSLv3 with Postfix releases
after the middle of 2015; older releases only disable SSLv2 for
mandatory TLS. The mandatory TLS protocol list is specified via the
smtpd_tls_mandatory_protocols configuration parameter. The
smtpd_tls_protocols parameter (Postfix ≥ 2.6)
controls the SSL/TLS protocols used with opportunistic TLS.

Note that the OpenSSL library only supports protocol exclusion
(not inclusion). For this reason, Postfix can exclude only protocols
that are known at the time the Postfix software is written. If new
protocols are added to the OpenSSL library, they cannot be excluded
without corresponding changes to the Postfix source code.

If you want to take maximal advantage of ciphers that offer forward secrecy see
the Getting
started section of FORWARD_SECRECY_README. The
full document conveniently presents all information about Postfix
"perfect" forward secrecy support in one place: what forward secrecy
is, how to tweak settings, and what you can expect to see when
Postfix uses ciphers with forward secrecy.

Postfix 2.8 and later, in combination with OpenSSL 0.9.7 and later
allows TLS servers to preempt the TLS client's cipher-suite preference list.
This is possible only with SSLv3 and later, as in SSLv2 the client
chooses the cipher-suite from a list supplied by the server.

By default, the OpenSSL server selects the client's most preferred
cipher-suite that the server supports. With SSLv3 and later, the server
may choose its own most preferred cipher-suite that is supported (offered)
by the client. Setting "tls_preempt_cipherlist = yes" enables server
cipher-suite preferences. The default OpenSSL behavior applies with
"tls_preempt_cipherlist = no".

While server cipher-suite selection may in some cases lead to
a more secure or performant cipher-suite choice, there is some risk
of interoperability issues. In the past, some SSL clients have
listed lower priority ciphers that they did not implement correctly.
If the server chooses a cipher that the client prefers less, it may
select a cipher whose client implementation is flawed. Most notably
Windows 2003 Microsoft Exchange servers have flawed implementations
of DES-CBC3-SHA, which OpenSSL considers stronger than RC4-SHA.
Enabling server cipher-suite selection may create interoperability
issues with Windows 2003 Microsoft Exchange clients.

With Postfix 2.8 and later, the tls_disable_workarounds parameter
specifies a list or bit-mask of OpenSSL bug work-arounds to disable. This
may be necessary if one of the work-arounds enabled by default in
OpenSSL proves to pose a security risk, or introduces an unexpected
interoperability issue. Some bug work-arounds known to be problematic
are disabled in the default value of the parameter when linked with
an OpenSSL library that could be vulnerable.

With Postfix ≥ 2.11, the tls_ssl_options parameter specifies
a list or bit-mask of OpenSSL options to enable. Specify one or
more of the named options below, or a hexadecimal bitmask of options
found in the ssl.h file corresponding to the run-time OpenSSL
library. While it may be reasonable to turn off all bug workarounds
(see above), it is not a good idea to attempt to turn on all features.

LEGACY_SERVER_CONNECT

See SSL_CTX_set_options(3).

NO_TICKET

See SSL_CTX_set_options(3).

NO_COMPRESSION

Disable SSL compression even if
supported by the OpenSSL library. Compression is CPU-intensive,
and compression before encryption does not always improve security.

You should only enable features via the hexadecimal mask when
the need to control the feature is critical (to deal with a new
vulnerability or a serious interoperability problem). Postfix DOES
NOT promise backwards compatible behavior with respect to the mask
bits. A feature enabled via the mask in one release may be enabled
by other means in a later release, and the mask bit will then be
ignored. Therefore, use of the hexadecimal mask is only a temporary
measure until a new Postfix or OpenSSL release provides a better
solution.

The smtp(8) and lmtp(8) delivery agents are implemented by a
single dual-purpose program. Specifically, all the TLS features
described below apply
equally to SMTP and LMTP, after replacing the "smtp_" prefix of the each
parameter name with "lmtp_".

The Postfix LMTP delivery agent can communicate with LMTP servers
listening
on UNIX-domain sockets. When server certificate verification is enabled
and the server is listening on a UNIX-domain socket, the $myhostname
parameter is used to set the TLS verification nexthop and
hostname.

NOTE: Opportunistic encryption of LMTP traffic over UNIX-domain
sockets or loopback TCP connections is futile. TLS is only useful
in this context when
it is mandatory, typically to allow at least one of the server or the
client to authenticate the other. The "null" cipher grade may be
appropriate in this context, when available on both client and server.
The "null" ciphers provide authentication without encryption.

At the "none" TLS security level, TLS encryption is
disabled. This is the default security level, and
can be configured explicitly by setting "smtp_tls_security_level = none".
For LMTP, use the corresponding "lmtp_" parameter.

Per-destination settings may override this default setting, in which case
TLS is used selectively, only with destinations explicitly configured
for TLS.

You can disable TLS for a subset of destinations, while leaving
it enabled for the rest. With the Postfix TLS policy table, specify the "none"
security level.

At the "may" TLS security level, TLS encryption is opportunistic.
The SMTP transaction is encrypted if the STARTTLS ESMTP feature
is supported by the server. Otherwise, messages are sent in the clear.
Opportunistic TLS can be configured by setting "smtp_tls_security_level = may".
For LMTP, use the corresponding "lmtp_" parameter.

The "smtp_tls_ciphers" and "smtp_tls_protocols" configuration
parameters (Postfix ≥ 2.6) provide control over the cipher grade
and protocols used with opportunistic TLS. With earlier Postfix
releases, opportunistic TLS always uses the cipher grade "export"
and enables all protocols.

With opportunistic TLS, mail delivery continues even if the
server certificate is untrusted or bears the wrong name.
When the TLS handshake fails for an opportunistic
TLS session, rather than give up on mail delivery, the Postfix SMTP
client retries the transaction
with TLS disabled. Trying an unencrypted connection makes
it possible to deliver mail to sites with non-interoperable server
TLS implementations.

Opportunistic encryption is never used for LMTP over UNIX-domain
sockets. The communications channel is already confidential without
TLS, so the only potential benefit of TLS is authentication. Do not
configure opportunistic TLS for LMTP deliveries over UNIX-domain sockets.
Only configure TLS for LMTP over UNIX-domain sockets at the
encrypt security level or higher.
Attempts to configure opportunistic encryption of LMTP sessions will
be ignored with a warning written to the mail logs.

You can enable opportunistic TLS just for selected destinations. With
the Postfix TLS policy table,
specify the "may" security level.

This is the most common security level for TLS protected SMTP
sessions, stronger security is not generally available and, if needed,
is typically only configured on a per-destination basis. See the section
on TLS limitations above.

At the "encrypt" TLS security level, messages are sent only
over TLS encrypted sessions. The SMTP transaction is aborted unless
the STARTTLS ESMTP feature is supported by the remote SMTP server.
If no suitable
servers are found, the message will be deferred.
Mandatory TLS encryption can be configured by setting
"smtp_tls_security_level = encrypt". Even though TLS
encryption is always used, mail delivery continues even if the server
certificate is untrusted or bears the wrong name.
For LMTP, use the corresponding "lmtp_" parameter.

At this security level and higher, the smtp_tls_mandatory_protocols
and smtp_tls_mandatory_ciphers configuration parameters determine
the list of sufficiently secure SSL protocol versions and the minimum
cipher strength. If the protocol or cipher requirements are not
met, the mail transaction is aborted. The documentation for these
parameters includes useful interoperability and security guidelines.

Despite the potential for eliminating passive eavesdropping attacks,
mandatory TLS encryption is not viable as a default security level for
mail delivery to the public Internet. Most MX hosts do not support TLS at
all, and some of those that do have broken implementations. On a host
that delivers mail to the Internet, you should not configure mandatory
TLS encryption as the default security level.

You can enable mandatory TLS encryption just for specific destinations.
With the Postfix TLS policy
table, specify the "encrypt" security level.

Examples:

In the example below, traffic to example.com and its sub-domains
via the corresponding MX hosts always uses TLS. The SSLv2 protocol
will be disabled (the default setting of smtp_tls_mandatory_protocols
excludes SSLv2+3). Only high- or medium-strength (i.e. 128 bit or
better) ciphers will be used by default for all "encrypt" security
level sessions.

In the next example, secure message submission is configured
via the MSA "[example.net]:587". TLS sessions are encrypted
without authentication, because this MSA does not possess an acceptable
certificate. This MSA is known to be capable of "TLSv1" and "high" grade
ciphers, so these are selected via the policy
table.

Note: the policy table lookup key is the verbatim next-hop
specification from the recipient domain, transport(5) table or relayhost
parameter, with any enclosing square brackets and optional port. Take
care to be consistent: the suffixes ":smtp" or ":25" or no port suffix
result in different policy table lookup keys, even though they are
functionally equivalent nexthop specifications. Use at most one of these
forms for all destinations. Below, the policy table has multiple keys,
just in case the transport table entries are not specified consistently.

The "dane" level is a stronger form of opportunistic TLS that is resistant to
man in the middle and downgrade attacks when the destination domain
uses DNSSEC to publish DANE TLSA records for its MX hosts. If a
remote SMTP server has "usable" (see RFC 6698) DANE TLSA records,
the server connection will be authenticated. When DANE authentication
fails, there is no fallback to unauthenticated or plaintext delivery.

If TLSA records are published for a given remote SMTP server
(implying TLS support), but are all "unusable" due to unsupported
parameters or malformed data, the Postfix SMTP client will use mandatory unauthenticated TLS.
Otherwise, when no TLSA records are published, the Postfix SMTP
client behavior is the same as with may.

TLSA records must be published in DNSSEC validated DNS zones.
Any TLSA records in DNS zones not protected via DNSSEC are ignored.
The Postfix SMTP client will not look for TLSA records associated
with MX hosts whose "A" or "AAAA" records lie in an "insecure" DNS
zone. Such lookups have been observed to cause interoperability
issues with poorly implemented DNS servers, and are in any case not
expected to ever yield "secure" results, since that would require
a very unlikely DLV DNS trust anchor configured between the host
record and the associated "_25._tcp" child TLSA record.

The "dane-only" level is a form of secure-channel TLS based on the DANE PKI.
If "usable" TLSA records are present these are used to authenticate the
remote SMTP server. Otherwise, or when server certificate verification
fails, delivery via the server in question tempfails.

At both security levels, the TLS policy for the destination is
obtained via TLSA records validated with DNSSEC. For TLSA policy
to be in effect, the destination domain's containing DNS zone must
be signed and the Postfix SMTP client's operating system must be
configured to send its DNS queries to a recursive DNS nameserver
that is able to validate the signed records. Each MX host's DNS
zone needs to also be signed, and needs to publish DANE TLSA (RFC 6698)
records that specify how that MX host's TLS certificate is to be
verified.

TLSA records do not preempt the normal SMTP MX host
selection algorithm, if some MX hosts support TLSA and others do
not, TLS security will vary from delivery to delivery. It is up
to the domain owner to configure their MX hosts and their DNS
sensibly. To configure the Postfix SMTP client for DNSSEC lookups
see the documentation for the smtp_dns_support_levelmain.cf
parameter. The tls_dane_trust_anchor_digest_enablemain.cf parameter
controls support for trust-anchor digest TLSA records. The
tls_dane_digests and tls_dane_digest_agility parameters control
the list of supported digests and digest downgrade attack resistance.

DANE for SMTP MTAs deviates in some details from the baseline
DANE protocol in RFC 6698. Most notably, it is not expected that
SMTP MTAs can reasonably include every public CA that a remote SMTP
server's administrator may believe to be well-known. Nor is there
an interactive user to "click OK" when authentication fails.

Therefore, certificate usages "0" and "1" from RFC 6698 which
are intended to "constrain" existing PKI trust, are not supported.
TLSA records with usage "0" are treated as "unusable". TLSA records
with usage "1" are instead treated as "trust assertions" and mapped
to usage "3". Specifically, with certificate usage "1", Postfix
will not require the remote SMTP server's certificate to be trusted
with respect to any locally defined public CAs, it is the domain
owner's responsibility to ensure that the certificate associations
in their TLSA records are appropriate to authenticate their SMTP
servers.

The Postfix SMTP client supports only certificate usages "2"
and "3" (with "1" treated as though it were "3"). See
tls_dane_trust_anchor_digest_enable for usage "2" usability
considerations. Support for certificate usage "1" is an experiment,
it may be withdrawn in the future. Server operators SHOULD NOT
publish TLSA records with usage "1".

When usable TLSA records are obtained for the remote SMTP server
the Postfix SMTP client sends the SNI TLS extension in its SSL
client hello message. This may help the remote SMTP server live
up to its promise to provide a certificate that matches its TLSA
records.

For purposes of protocol and cipher selection, the "dane"
security level is treated like a "mandatory" TLS security level,
and weak ciphers and protocols are disabled. Since DANE authenticates
server certificates the "aNULL" cipher-suites are transparently
excluded at this level, no need to configure this manually. RFC6698 (DANE) TLS authentication is available with Postfix 2.11 and
later.

When a DANE TLSA record specifies a trust-anchor (TA) certificate
(that is an issuing CA), the strategy used to verify the peername
of the server certificate is unconditionally "nexthop, hostname".
Both the nexthop domain and the hostname obtained from the
DNSSEC-validated MX lookup are safe from forgery and the server
certificate must contain at least one of these names.

When a DANE TLSA record specifies an end-entity (EE) certificate,
(that is the actual server certificate), as with the fingerprint
security level below, no name checks or certificate expiration checks
are applied. The server certificate (or its public key) either matches
the DANE record or not. Server administrators should publish such
EE records in preference to all other types.

The above client pre-requisites do not apply to the Postfix SMTP server.
It will support DANE provided it supports TLSv1 and its TLSA records are
published in a DNSSEC signed zone. To receive DANE secured mail for multiple
domains, use the same hostname to add the server to each domain's MX
records. There are no plans to implement SNI in the Postfix SMTP server.

Note: The Postfix SMTP client's internal stub DNS resolver is
DNSSEC-aware, but it does not itself validate DNSSEC records, rather
it delegates DNSSEC validation to the operating system's configured
recursive DNS nameserver. The Postfix DNS client relies on a secure
channel to the resolver's cache for DNSSEC integrity, but does not
support TSIG to protect the transmission channel between itself and
the nameserver. Therefore, it is strongly recommended (DANE security
guarantee void otherwise) that each MTA run a local DNSSEC-validating
recursive resolver ("unbound" from nlnetlabs.nl is a reasonable
choice) listening on the loopback interface, and that the system
be configured to use only this local nameserver. The local
nameserver may forward queries to an upstream recursive resolver
on another host if desired.

Note: When the operating system's recursive nameserver is not
local, enabling EDNS0 expanded DNS packet sizes and turning on the
DNSSEC "DO" bit in the DNS request and/or the new DNSSEC-specific
records returned in the nameserver's replies may cause problems
with older or buggy firewall and DNS server implementations.
Therefore, Postfix does not enable DNSSEC by default. Since MX
lookups happen before the security level is determined, DANE support
is disabled for all destinations unless you set "smtp_dns_support_level
= dnssec". To enable DNSSEC lookups selectively, define a new
dedicated transport with a "-o smtp_dns_support_level=dnssec"
override in master.cf and route selected domains to that transport.
If DNSSEC proves to be sufficiently reliable for these domains, you
can enable it for all destinations by changing the global
smtp_dns_support_level in main.cf.

Example: "dane" security for selected destinations, with
opportunistic TLS by default. This is the recommended configuration
for early adopters.

The "example.com" destination uses DANE, but if TLSA records
are not present or are unusable, mail is deferred.

The "example.org" destination uses DANE if possible, but if no TLSA
records are found opportunistic TLS is used.

At the fingerprint security level, no trusted Certification
Authorities are used or required. The certificate trust chain,
expiration date, etc., are not checked. Instead, the
smtp_tls_fingerprint_cert_match parameter or the "match" attribute
in the policy table lists the
remote SMTP server certificate fingerprint or public key fingerprint.
Certificate fingerprint verification is available with Postfix 2.5
and later, public-key fingerprint support is available with Postfix
2.9 and later.

If certificate fingerprints are exchanged securely, this is the
strongest, and least scalable security level. The administrator needs
to securely collect the fingerprints of the X.509 certificates of each
peer server, store them into a local file, and update this local file
whenever the peer server's public certificate changes. If public key
fingerprints are used in place of fingerprints of the entire certificate,
the fingerprints remain valid even after the certificate is renewed,
provided that the same public/private keys are used to obtain
the new certificate.

Fingerprint verification may be feasible for an SMTP "VPN" connecting
a small number of branch offices over the Internet, or for secure
connections to a central mail hub. It works poorly if the remote SMTP
server is managed by a third party, and its public certificate changes
periodically without prior coordination with the verifying site.

The digest algorithm used to calculate the fingerprint is
selected by the smtp_tls_fingerprint_digest parameter. In the policy table multiple fingerprints can be
combined with a "|" delimiter in a single match attribute, or multiple
match attributes can be employed. The ":" character is not used as a
delimiter as it occurs between each pair of fingerprint (hexadecimal)
digits.

Example: fingerprint TLS security with an internal mailhub.
Two matching fingerprints are listed. The relayhost may be multiple
physical hosts behind a load-balancer, each with its own private/public
key and self-signed certificate. Alternatively, a single relayhost may
be in the process of switching from one set of private/public keys to
another, and both keys are trusted just prior to the transition.

To extract the public key fingerprint from an X.509 certificate,
you need to extract the public key from the certificate and compute
the appropriate digest of its DER (ASN.1) encoding. With OpenSSL
the "-pubkey" option of the "x509" command extracts the public
key always in "PEM" format. We pipe the result to another OpenSSL
command that converts the key to DER and then to the "dgst" command
to compute the fingerprint.

The actual command to transform the key to DER format depends
on the version of OpenSSL used. With OpenSSL 1.0.0 and later, the
"pkey" command supports all key types. With OpenSSL 0.9.8 and
earlier, the key type is always RSA (nobody uses DSA, and EC
keys are not fully supported by 0.9.8), so the "rsa" command is
used.

At the verify TLS security level, messages are sent only over
TLS encrypted sessions if the remote SMTP server certificate is
valid (not
expired or revoked, and signed by a trusted Certification Authority)
and where the server certificate name matches a known pattern.
Mandatory
server certificate verification can be configured by setting
"smtp_tls_security_level = verify". The
smtp_tls_verify_cert_match parameter can override the default
"hostname" certificate name matching strategy. Fine-tuning the
matching strategy is generally only appropriate for secure-channel destinations.
For LMTP use the corresponding "lmtp_" parameters.

If the server certificate chain is trusted (see smtp_tls_CAfile
and smtp_tls_CApath), any DNS names in the SubjectAlternativeName
certificate extension are used to verify the remote SMTP server name.
If no
DNS names are specified, the certificate CommonName is checked.
If you want mandatory encryption without server certificate
verification, see above.

With Postfix ≥ 2.11 the "smtp_tls_trust_anchor_file" parameter
or more typically the corresponding per-destination "tafile" attribute
optionally modifies trust chain verification. If the parameter is
not empty the root CAs in CAfile and CApath are no longer trusted.
Rather, the Postfix SMTP client will only trust certificate-chains
signed by one of the trust-anchors contained in the chosen files.
The specified trust-anchor certificates and public keys are not
subject to expiration, and need not be (self-signed) root CAs. They
may, if desired, be intermediate certificates. Therefore, these
certificates also may be found "in the middle" of the trust chain
presented by the remote SMTP server, and any untrusted issuing
parent certificates will be ignored.

Despite the potential for eliminating "man-in-the-middle" and other
attacks, mandatory certificate trust chain and subject name verification
is not viable as a default Internet mail delivery policy. Most MX hosts
do not support TLS at all, and a significant portion of TLS enabled
MTAs use self-signed certificates, or certificates that are signed by
a private Certification Authority. On a machine that delivers mail to
the Internet, you should not configure mandatory server certificate
verification as a default policy.

Mandatory server certificate verification as a default security
level may be appropriate if you know that you will only connect to
servers that support RFC 2487and that present verifiable
server certificates. An example would be a client that sends all
email to a central mailhub that offers the necessary STARTTLS
support. In such cases, you can often use a secure-channel configuration instead.

You can enable mandatory server certificate verification just
for specific destinations. With the Postfix TLS policy table, specify the "verify"
security level.

Example:

In this example, the Postfix SMTP client encrypts all traffic to the
example.com domain. The peer hostname is verified, but
verification is vulnerable to DNS response forgery. Mail transmission
to example.com recipients uses "high" grade ciphers.

At the secure TLS security level, messages are sent only over
secure-channel TLS sessions where DNS forgery resistant server
certificate verification succeeds. If no suitable servers are found, the
message will be deferred. Postfix secure-channels
can be configured by setting "smtp_tls_security_level = secure".
The smtp_tls_secure_cert_match parameter can override the default
"nexthop, dot-nexthop" certificate match strategy.
For LMTP, use the corresponding "lmtp_" parameters.

If the server certificate chain is trusted (see smtp_tls_CAfile and
smtp_tls_CApath), any DNS names in the SubjectAlternativeName certificate
extension are used to verify the remote SMTP server name. If no DNS names
are
specified, the CommonName is checked. If you want mandatory encryption
without server certificate verification, see above.

With Postfix ≥ 2.11 the "smtp_tls_trust_anchor_file" parameter
or more typically the corresponding per-destination "tafile" attribute
optionally modifies trust chain verification. If the parameter is
not empty the root CAs in CAfile and CApath are no longer trusted.
Rather, the Postfix SMTP client will only trust certificate-chains
signed by one of the trust-anchors contained in the chosen files.
The specified trust-anchor certificates and public keys are not
subject to expiration, and need not be (self-signed) root CAs. They
may, if desired, be intermediate certificates. Therefore, these
certificates also may be found "in the middle" of the trust chain
presented by the remote SMTP server, and any untrusted issuing
parent certificates will be ignored.

Despite the potential for eliminating "man-in-the-middle" and other
attacks, mandatory secure server certificate verification is not
viable as a default Internet mail delivery policy. Most MX hosts
do not support TLS at all, and a significant portion of TLS enabled
MTAs use self-signed certificates, or certificates that are signed
by a private Certification Authority. On a machine that delivers mail
to the Internet, you should not configure secure TLS verification
as a default policy.

Mandatory secure server certificate verification as a default
security level may be appropriate if you know that you will only
connect to servers that support RFC 2487and that present
verifiable server certificates. An example would be a client that
sends all email to a central mailhub that offers the necessary
STARTTLS support.

You can enable secure TLS verification just for specific destinations.
With the Postfix TLS policy table,
specify the "secure" security level.

The Postfix SMTP client will encrypt all traffic and verify the
destination name
immune from forged DNS responses. MX lookups are still used to find
the hostnames of the SMTP servers for example.com, but these
hostnames are not used when
checking the names in the server certificate(s). Rather, the requirement
is that the MX hosts for example.com have trusted certificates
with a subject name of example.com or a sub-domain, see the
documentation for the smtp_tls_secure_cert_match parameter.

The related domains example.co.uk and example.co.jp are
hosted on the same MX hosts as the primary example.com domain, and
traffic to these is secured by verifying the primary example.com
domain in the server certificates. This frees the server administrator
from needing the CA to sign certificates that list all the secondary
domains. The downside is that clients that want secure channels to the
secondary domains need explicit TLS policy
table entries.

Note, there are two ways to handle related domains. The first is to
use the default routing for each domain, but add policy table entries
to override the expected certificate subject name. The second is to
override the next-hop in the transport table, and use a single policy
table entry for the common nexthop. We choose the first approach,
because it works better when domain ownership changes. With the second
approach we securely deliver mail to the wrong destination, with the
first approach, authentication fails and mail stays in the local queue,
the first approach is more appropriate in most cases.

In this case traffic to example.com and its related domains
is sent to a single logical gateway (to avoid a single point of failure,
its name may resolve to one or more load-balancer addresses, or to the
combined addresses of multiple physical hosts). All the physical hosts
reachable via the gateway's IP addresses have the logical gateway name
listed in their certificates.

Do not configure Postfix SMTP client certificates unless you must
present
client TLS certificates to one or more servers. Client certificates are
not usually needed, and can cause problems in configurations that work
well without them. The recommended setting is to let the defaults stand:

The best way to use the default settings is to comment out the above
parameters in main.cf if present.

During TLS startup negotiation the Postfix SMTP client may present
a certificate to the remote SMTP server. The Netscape client is
rather clever here and lets the user select between only those
certificates that match CA certificates offered by the remote SMTP
server. As the Postfix SMTP client uses the "SSL_connect()" function
from the OpenSSL package, this is not possible and we have to choose
just one certificate. So for now the default is to use _no_
certificate and key unless one is explicitly specified here.

RSA, DSA and ECDSA (Postfix ≥ 2.6) certificates are supported.
You can configure all three at the same time, in which case the
cipher used determines which certificate is presented.

It is possible for the Postfix SMTP client to use the same
key/certificate pair as the Postfix SMTP server. If a certificate
is to be presented, it must be in "PEM" format. The private key
must not be encrypted, meaning: it must be accessible without
password. Both parts (certificate and private key) may be in the
same file.

To enable remote SMTP servers to verify the Postfix SMTP client
certificate, the issuing CA certificates must be made available to the
server. You should include the required certificates in the client
certificate file, the client certificate first, then the issuing
CA(s) (bottom-up order).

Example: the certificate for "client.example.com" was issued by
"intermediate CA" which itself has a certificate issued by "root CA".
Create the client.pem file with:

A server that trusts the root CA has a local copy of the root
CA certificate, so it is not necessary to include the root CA
certificate here. Leaving it out of the "client.pem" file reduces
the overhead of the TLS exchange.

If you want the Postfix SMTP client to accept remote SMTP server
certificates issued by these CAs, append the root certificate to
$smtp_tls_CAfile or install it in the $smtp_tls_CApath directory.

To verify a remote SMTP server certificate, the Postfix SMTP
client needs to trust the certificates of the issuing Certification
Authorities. These certificates in "pem" format can be stored in a
single $smtp_tls_CAfile or in multiple files, one CA per file in
the $smtp_tls_CApath directory. If you use a directory, don't forget
to create the necessary "hash" links with:

# $OPENSSL_HOME/bin/c_rehash /path/to/directory

The $smtp_tls_CAfile contains the CA certificates of one or more
trusted CAs. The file is opened (with root privileges) before Postfix
enters the optional chroot jail and so need not be accessible from inside the
chroot jail.

Additional trusted CAs can be specified via the $smtp_tls_CApath
directory, in which case the certificates are read (with $mail_owner
privileges) from the files in the directory when the information
is needed. Thus, the $smtp_tls_CApath directory needs to be accessible
inside the optional chroot jail.

The choice between $smtp_tls_CAfile and $smtp_tls_CApath is
a space/time tradeoff. If there are many trusted CAs, the cost of
preloading them all into memory may not pay off in reduced access time
when the certificate is needed.

The remote SMTP server and the Postfix SMTP client negotiate a
session, which takes some computer time and network bandwidth. By
default, this session information is cached only in the smtp(8)
process actually using this session and is lost when the process
terminates. To share the session information between multiple
smtp(8) processes, a persistent session cache can be used. You
can specify any database type that can store objects of several
kbytes and that supports the sequence operator. DBM databases are
not suitable because they can only store small objects. The cache
is maintained by the tlsmgr(8) process, so there is no problem with
concurrent access. Session caching is highly recommended, because
the cost of repeatedly negotiating TLS session keys is high. Future
Postfix SMTP servers may limit the number of sessions that a client
is allowed to negotiate per unit time.

Note: as of version 2.5, Postfix no longer uses root privileges
when opening this file. The file should now be stored under the
Postfix-owned data_directory. As a migration aid, an attempt to
open the file under a non-Postfix directory is redirected to the
Postfix-owned data_directory, and a warning is logged.

Cached Postfix SMTP client session information expires after
a certain amount of time. Postfix/TLS does not use the OpenSSL
default of 300s, but a longer time of 3600s (=1 hour). RFC 2246
recommends a maximum of 24 hours.

The security properties of TLS communication channels are
application specific. While the TLS protocol can provide a confidential,
tamper-resistant, mutually authenticated channel between client
and server, not all of these security features are applicable to every
communication.

For example, while mutual TLS authentication between browsers and web
servers is possible, it is not practical, or even useful, for web-servers
that serve the public to verify the identity of every potential user. In
practice, most HTTPS transactions are asymmetric: the browser verifies
the HTTPS server's identity, but the user remains anonymous. Much of
the security policy is up to the client. If the client chooses to not
verify the server's name, the server is not aware of this. There are many
interesting browser security topics, but we shall not dwell
on them here. Rather, our goal is to understand the security features
of TLS in conjunction with SMTP.

An important SMTP-specific observation is that a public MX host is
even more at the mercy of the SMTP client than is an HTTPS server. Not only
can it not enforce due care in the client's use of TLS, but it cannot even
enforce the use of TLS, because TLS support in SMTP clients is still the
exception rather than the rule. One cannot, in practice, limit access to
one's MX hosts to just TLS-enabled clients. Such a policy would result
in a vast reduction in one's ability to communicate by email with the
world at large.

One may be tempted to try enforcing TLS for mail from specific
sending organizations, but this, too, runs into obstacles. One such
obstacle is that we don't know who is (allegedly) sending mail until
we see the "MAIL FROM:" SMTP command, and at that point, if TLS
is not already in use, a potentially sensitive sender address (and
with SMTP PIPELINING one or more of the recipients) has (have) already been
leaked in the clear. Another obstacle is that mail from the sender to
the recipient may be forwarded, and the forwarding organization may not
have any security arrangements with the final destination. Bounces also
need to be protected. These can only be identified by the IP address and
HELO name of the connecting client, and it is difficult to keep track
of all the potential IP addresses or HELO names of the outbound email
servers of the sending organization.

Consequently, TLS security for mail delivery to public MX hosts is
almost entirely the client's responsibility. The server is largely a
passive enabler of TLS security, the rest is up to the client. While the
server has a greater opportunity to mandate client security policy when
it is a dedicated MSA that only handles outbound mail from trusted clients,
below we focus on the client security policy.

On the SMTP client, there are further complications. When
delivering mail to a given domain, in contrast to HTTPS, one rarely
uses the domain name directly as the target host of the SMTP session.
More typically, one uses MX lookups — these are usually
unauthenticated — to obtain the domain's SMTP server hostname(s).
When, as is current practice, the client verifies the insecurely
obtained MX hostname, it is subject to a DNS man-in-the-middle
attack.

Adoption of DNSSEC and RFC6698 (DANE) may gradually (as domains
implement DNSSEC and publish TLSA records for their MX hosts) address
the DNS man-in-the-middle risk and provide scalable key management
for SMTP with TLS. Postfix ≥ 2.11 supports the new dane and dane-only
security levels that take advantage of these standards.

If clients instead attempted to verify the recipient domain name,
an SMTP server for multiple domains would need to
list all its email domain names in its certificate, and generate a
new certificate each time a new domain were added. At least some CAs set
fairly low limits (20 for one prominent CA) on the number of names that
server certificates can contain. This approach is not consistent with
current practice and does not scale.

It is regrettably the case that TLS secure-channels
(fully authenticated and immune to man-in-the-middle attacks) impose
constraints on the sending and receiving sites that preclude ubiquitous
deployment. One needs to manually configure this type of security for
each destination domain, and in many cases implement non-default TLS
policy table entries for additional
domains hosted at a common secured destination. For these reasons
secure-channel configurations
will never be the norm. For the generic domain with which you
have made no specific security arrangements, this security level is not
a good fit.

Given that strong authentication is not generally possible, and that
verifiable certificates cost time and money, many servers that implement
TLS use self-signed certificates or private CAs. This further limits
the applicability of verified TLS on the public Internet.

Historical note: while the documentation of these issues and many of the
related features were new with Postfix 2.3, the issue was well
understood before Postfix 1.0, when Lutz Jänicke was designing
the first unofficial Postfix TLS patch. See his original post http://www.imc.org/ietf-apps-tls/mail-archive/msg00304.html
and the first response http://www.imc.org/ietf-apps-tls/mail-archive/msg00305.html.
The problem is not even unique to SMTP or even TLS, similar issues exist
for secure connections via aliases for HTTPS and Kerberos. SMTP merely
uses indirect naming (via MX records) more frequently.

A small fraction of servers offer STARTTLS but the negotiation
consistently fails. As long as encryption is not mandatory, the
Postfix SMTP client retries the delivery immediately with TLS
disabled, without any need to explicitly disable TLS for the problem
destinations.

The TLS policy table is indexed by the full next-hop destination,
which is either the recipient domain, or the verbatim next-hop
specified in the transport table, $local_transport, $virtual_transport,
$relay_transport or $default_transport. This includes any enclosing
square brackets and any non-default destination server port suffix. The
LMTP socket type prefix (inet: or unix:)
is not included in the lookup key.

Only the next-hop domain, or $myhostname with LMTP over UNIX-domain
sockets, is used as the nexthop name for certificate verification. The
port and any enclosing square brackets are used in the table lookup key,
but are not used for server name verification.

When the lookup key is a domain name without enclosing square brackets
or any :port suffix (typically the recipient domain), and the full
domain is not found in the table, just as with the transport(5) table,
the parent domain starting with a leading "." is matched recursively. This
allows one to specify a security policy for a recipient domain and all
its sub-domains.

The lookup result is a security level, followed by an optional
list of whitespace and/or comma separated name=value attributes
that override related main.cf settings. The TLS security levels are described above. Below, we
describe the corresponding table syntax:

Opportunistic DANE TLS.
The TLS policy for the destination is obtained via TLSA records in
DNSSEC. If no TLSA records are found, the effective security level
used is may. If TLSA records are
found, but none are usable, the effective security level is encrypt. When usable TLSA records
are obtained for the remote SMTP server, SSLv2+3 are automatically
disabled (see smtp_tls_mandatory_protocols), and the server certificate
must match the TLSA records. RFC 6698 (DANE) TLS authentication
and DNSSEC support is available with Postfix 2.11 and later.

dane-only

Mandatory DANE TLS.
The TLS policy for the destination is obtained via TLSA records in
DNSSEC. If no TLSA records are found, or none are usable, no
connection is made to the server. When usable TLSA records are
obtained for the remote SMTP server, SSLv2+3 are automatically disabled
(see smtp_tls_mandatory_protocols), and the server certificate must
match the TLSA records. RFC 6698 (DANE) TLS authentication and
DNSSEC support is available with Postfix 2.11 and later.

fingerprint

Certificate
fingerprint verification. Available with Postfix 2.5 and
later. At this security level, there are no trusted Certification
Authorities. The certificate trust chain, expiration date, ... are
not checked. Instead, the optional match attribute, or else
the main.cfsmtp_tls_fingerprint_cert_match parameter, lists
the server certificate fingerprints or public key fingerprints
(Postfix 2.9 and later). The
digest algorithm used to calculate fingerprints is selected by the
smtp_tls_fingerprint_digest parameter. Multiple fingerprints can
be combined with a "|" delimiter in a single match attribute, or multiple
match attributes can be employed. The ":" character is not used as a
delimiter as it occurs between each pair of fingerprint (hexadecimal)
digits.

verify

Mandatory
server certificate verification. Mail is delivered only if the
TLS handshake succeeds, if the remote SMTP server certificate can
be validated (not expired or revoked, and signed by a trusted
Certification Authority), and if the server certificate name matches
the optional "match" attribute (or the main.cfsmtp_tls_verify_cert_match
parameter value when no optional "match" attribute is specified).
With Postfix ≥ 2.11 the "tafile" attribute optionally modifies
trust chain verification in the same manner as the
"smtp_tls_trust_anchor_file" parameter. The "tafile" attribute
may be specified multiple times to load multiple trust-anchor
files.

secure

Secure certificate
verification. Mail is delivered only if the TLS handshake succeeds,
if the remote SMTP server certificate can be validated (not expired
or revoked, and signed by a trusted Certification Authority), and if the
server certificate name matches the optional "match" attribute (or the
main.cfsmtp_tls_secure_cert_match parameter value when no optional
"match" attribute is specified). With Postfix ≥ 2.11 the "tafile"
attribute optionally modifies trust chain verification in the same manner
as the "smtp_tls_trust_anchor_file" parameter. The "tafile" attribute
may be specified multiple times to load multiple trust-anchor
files.

Notes:

The "match" attribute is especially useful to verify TLS
certificates for domains that are hosted on a shared server. In
that case, specify "match" rules for the shared server's name.
While secure verification can also be achieved with manual routing
overrides in Postfix transport(5) tables, that approach can deliver
mail to the wrong host when domains are assigned to new gateway
hosts. The "match" attribute approach avoids the problems of manual
routing overrides; mail is deferred if verification of a new MX
host fails.

When a policy table entry specifies multiple match patterns,
multiple match strategies, or multiple protocols, these must be
separated by colons.

The "exclude" attribute (Postfix ≥ 2.6) is used to disable
ciphers that cause handshake failures with a specific mandatory TLS
destination, without disabling the ciphers for all mandatory destinations.
Alternatively, you can exclude ciphers that cause issues with multiple
remote servers in main.cf, and selectively enable them on a per-destination
basis in the policy table by setting a shorter or empty exclusion list. The
per-destination "exclude" list preempts both the opportunistic and
mandatory security level exclusions, so that all excluded ciphers
can be enabled for known-good destinations. For non-mandatory TLS
destinations that exhibit cipher-specific problems, Postfix will fall
back to plain-text delivery. If plain-text is not acceptable make TLS
mandatory and exclude the problem ciphers.

Note: The "hostname" strategy if listed in a non-default setting
of smtp_tls_secure_cert_match or in the "match" attribute in the policy
table can render the "secure" level vulnerable to DNS forgery. Do not use
the "hostname" strategy for secure-channel
configurations in environments where DNS security is not assured.

The server certificate verification depth is specified with the
main.cfsmtp_tls_scert_verifydepth parameter. The default verification
depth is 9 (the OpenSSL default), for compatibility with Postfix
versions before 2.5 where smtp_tls_scert_verifydepth was ignored.
When you configure trust
in a root CA, it is not necessary to explicitly trust intermediary CAs
signed by the root CA, unless $smtp_tls_scert_verifydepth is less than the
number of CAs in the certificate chain for the servers of interest. With
a verify depth of 1 you can only verify certificates directly signed
by a trusted CA, and all trusted intermediary CAs need to be configured
explicitly. With a verify depth of 2 you can verify servers signed by a
root CA or a direct intermediary CA (so long as the server is correctly
configured to supply its intermediate CA certificate).

The Postfix SMTP client supports 5 distinct cipher grades
as specified by the smtp_tls_mandatory_ciphers configuration
parameter. This setting controls the minimum acceptable SMTP client
TLS cipher grade for use with mandatory TLS encryption. The default
value "medium" is suitable for most destinations with which you may
want to enforce TLS, and is beyond the reach of today's cryptanalytic
methods. See smtp_tls_policy_maps for information on how to configure
ciphers on a per-destination basis.

By default anonymous ciphers are allowed, and automatically
disabled when remote SMTP server certificates are verified. If you
want to
disable anonymous ciphers even at the "encrypt" security level, set
"smtp_tls_mandatory_exclude_ciphers = aNULL"; and to
disable anonymous ciphers even with opportunistic TLS, set
"smtp_tls_exclude_ciphers = aNULL". There is generally
no need to take these measures. Anonymous ciphers save bandwidth
and TLS session cache space, if certificates are ignored, there is
little point in requesting them.

The "smtp_tls_ciphers" configuration parameter (Postfix ≥ 2.6)
provides control over the minimum cipher grade for opportunistic TLS.
The default minimum cipher grade for opportunistic TLS is "medium"
for Postfix releases after the middle of 2015, and "export" for
older releases. With Postfix < 2.6, the minimum opportunistic
TLS cipher grade is always "export".

These sections show how to send mail to a server that does not
support STARTTLS, but that provides the deprecated SMTPS service
on TCP port 465. Depending on the Postfix version, some additional
tooling may be required.

Postfix ≥ 3.0

The Postfix SMTP client has SMTPS support built-in as of version
3.0. Use one of the following examples, to send all remote mail,
or to send only some remote mail, to an SMTPS server.

Postfix ≥ 3.0: Sending all remote mail to an SMTPS server

The first example will send all remote mail over SMTPS through
a provider's server called "mail.example.com":

Postfix < 3.0

Although older Postfix SMTP client versions do not support TLS
wrapper mode, it is relatively easy to forward a connection through
the stunnel program if Postfix needs to deliver mail to some legacy
system that doesn't support STARTTLS.

Postfix < 3.0: Sending all remote mail to an SMTPS server

The first example uses SMTPS to send all remote mail to a
provider's mail server called "mail.example.com".

A minimal stunnel.conf file is sufficient to set up a tunnel
from local port 11125 to the remote destination "mail.example.com"
and port "smtps". Postfix will later use this tunnel to connect to
the remote server.

The smtp_starttls_timeout parameter limits the time of Postfix
SMTP client write and read operations during TLS startup and shutdown
handshake procedures. In case of problems the Postfix SMTP client
tries the next network address on the mail exchanger list, and
defers delivery if no alternative server is available.

With Postfix 2.8 and later, the tls_disable_workarounds parameter
specifies a list or bit-mask of OpenSSL bug work-arounds to disable. This
may be necessary if one of the work-arounds enabled by default in
OpenSSL proves to pose a security risk, or introduces an unexpected
interoperability issue. Some bug work-arounds known to be problematic
are disabled in the default value of the parameter when linked with
an OpenSSL library that could be vulnerable.

Note: Disabling LEGACY_SERVER_CONNECT is not wise at this
time, lots of servers are still unpatched and Postfix is not
significantly vulnerable to the renegotiation issue in the TLS
protocol.

With Postfix ≥ 2.11, the tls_ssl_options parameter specifies
a list or bit-mask of OpenSSL options to enable. Specify one or
more of the named options below, or a hexadecimal bitmask of options
found in the ssl.h file corresponding to the run-time OpenSSL
library. While it may be reasonable to turn off all bug workarounds
(see above), it is not a good idea to attempt to turn on all features.

A future version of OpenSSL may by default no longer allow
connections to servers that don't support secure renegotiation.
Since the exposure for SMTP is minimal, and some SMTP servers may
remain unpatched, you can add LEGACY_SERVER_CONNECT to the
options to restore the more permissive default of current OpenSSL
releases.

You should only enable features via the hexadecimal mask when
the need to control the feature is critical (to deal with a new
vulnerability or a serious interoperability problem). Postfix DOES
NOT promise backwards compatible behavior with respect to the mask
bits. A feature enabled via the mask in one release may be enabled
by other means in a later release, and the mask bit will then be
ignored. Therefore, use of the hexadecimal mask is only a temporary
measure until a new Postfix or OpenSSL release provides a better
solution.

The security of cryptographic software such as TLS depends
critically on the ability to generate unpredictable numbers for
keys and other information. To this end, the tlsmgr(8) process
maintains a Pseudo Random Number Generator (PRNG) pool. This is
queried by the smtp(8) and smtpd(8) processes when they initialize.
By default, these daemons request 32 bytes, the equivalent to 256
bits. This is more than sufficient to generate a 128bit (or 168bit)
session key.

In order to feed its in-memory PRNG pool, the tlsmgr(8) reads
entropy from an external source, both at startup and during run-time.
Specify a good entropy source, like EGD or /dev/urandom; be sure
to only use non-blocking sources (on OpenBSD, use /dev/arandom
when tlsmgr(8) complains about /dev/urandom timeout errors).
If the entropy source is not a
regular file, you must prepend the source type to the source name:
"dev:" for a device special file, or "egd:" for a source with EGD
compatible socket interface.

By default, tlsmgr(8) reads 32 bytes from the external entropy
source at each seeding event. This amount (256bits) is more than
sufficient for generating a 128bit symmetric key. With EGD and
device entropy sources, the tlsmgr(8) limits the amount of data
read at each step to 255 bytes. If you specify a regular file as
entropy source, a larger amount of data can be read.

In order to update its in-memory PRNG pool, the tlsmgr(8)
queries the external entropy source again after a pseudo-random
amount of time. The time is calculated using the PRNG, and is
between 0 and the maximal time specified with tls_random_reseed_period.
The default maximal time interval is 1 hour.

The tlsmgr(8) process saves the PRNG state to a persistent
exchange file at regular times and when the process terminates, so
that it can recover the PRNG state the next time it starts up.
This file is created when it does not exist.

As of version 2.5, Postfix no longer uses root privileges when
opening this file. The file should now be stored under the Postfix-owned
data_directory. As a migration aid, an attempt to open the file
under a non-Postfix directory is redirected to the Postfix-owned
data_directory, and a warning is logged. If you wish to continue
using a pre-existing PRNG state file, move it to the data_directory
and change the ownership to the account specified with the mail_owner
parameter.

With earlier Postfix versions the default file location
is under the Postfix configuration directory, which is not the
proper place for information that is modified by Postfix.

The following steps will get you started quickly. Because you
sign your own Postfix public key certificate, you get TLS encryption
but no TLS authentication. This is sufficient for testing, and
for exchanging email with sites that you have no trust relationship
with. For real authentication, your Postfix public key certificate
needs to be signed by a recognized Certification Authority, and
Postfix needs to be configured with a list of public key certificates
of Certification Authorities, so that Postfix can verify the public key
certificates of remote hosts.

In the examples below, user input is shown in bold
font, and a "#" prompt indicates a super-user shell.

Note: the last command requires both single (') and double (")
quotes.

The postconf(1) command above enables opportunistic TLS for
receiving and sending mail. It also enables logging of TLS connections
and recording of TLS use in the "Received" header. TLS session
caching is also enabled in the Postfix SMTP client. With Postfix
≥ 2.10, the SMTP server does not need an explicit session cache
since session reuse is better handled via RFC 5077 TLS session
tickets.

Become your own Certification Authority, so that you can
sign your own certificates, and so that your own systems can
authenticate certificates from your own CA. This example uses the
CA.pl script that ships with OpenSSL. On some systems, OpenSSL
installs this as /usr/local/openssl/misc/CA.pl. Some systems
install this as
part of a package named openssl-perl or something similar.
The script creates a private key in ./demoCA/private/cakey.pem
and a public key in ./demoCA/cacert.pem.

Create an unpassworded private key for host foo.porcupine.org and create
an unsigned public key certificate.

% (umask 077; openssl req -new -newkey rsa:2048 -nodes -keyout foo-key.pem -out foo-req.pem)
Using configuration from /etc/ssl/openssl.cnf
Generating a 2048 bit RSA private key
........................................++++++
....++++++
writing new private key to 'foo-key.pem'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:New York
Locality Name (eg, city) []:Westchester
Organization Name (eg, company) [Internet Widgits Pty Ltd]:Porcupine
Organizational Unit Name (eg, section) []:
Common Name (eg, YOUR name) []:foo.porcupine.org
Email Address []:wietse@porcupine.org
Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:whatever
An optional company name []:

Sign the public key certificate for host foo.porcupine.org with the
Certification Authority private key that we created a few
steps ago.

Configure Postfix, by adding the following to
/etc/postfix/main.cf. It is generally best to not configure
client certificates, unless there are servers which authenticate your mail
submission via client certificates. Often servers that perform TLS client
authentication will issue the required certificates signed by their own
CA. If you configure the client certificate and key incorrectly, you
will be unable to send mail to sites that request client certificate,
but don't require them from all clients.

These instructions assume that you build Postfix from source
code as described in the INSTALL document. Some modification may
be required if you build Postfix from a vendor-specific source
package.

To build Postfix with TLS support, first we need to generate
the make(1) files with the necessary definitions. This is
done by invoking the command "make makefiles" in the Postfix
top-level directory and with arguments as shown next.

NOTE: Do not use Gnu TLS. It will spontaneously terminate
a Postfix daemon process with exit status code 2, instead of allowing
Postfix to 1) report the error to the maillog file, and to 2) provide
plaintext service where this is appropriate.

If the OpenSSL include files (such as ssl.h) are
in directory /usr/include/openssl, and the OpenSSL libraries
(such as libssl.so and libcrypto.so) are in
directory /usr/lib:

% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_TLS" AUXLIBS="-lssl -lcrypto"

If the OpenSSL include files (such as ssl.h) are
in directory /usr/local/include/openssl, and the OpenSSL
libraries (such as libssl.so and libcrypto.so)
are in directory /usr/local/lib:

% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \
AUXLIBS="-L/usr/local/lib -lssl -lcrypto"

On Solaris, specify the -R option as shown below:

% make tidy # if you have left-over files from a previous build
% make makefiles CCARGS="-DUSE_TLS -I/usr/local/include" \
AUXLIBS="-R/usr/local/lib -L/usr/local/lib -lssl -lcrypto"

If you need to apply other customizations (such as Berkeley DB
databases, MySQL, PostgreSQL, LDAP or SASL), see the respective
Postfix README documents, and combine their "make makefiles"
instructions with the instructions above:

Problems are preferably reported via <postfix-users@postfix.org>.
See http://www.postfix.org/lists.html for subscription information.
When reporting a problem, please be thorough in the report. Patches,
when possible, are greatly appreciated too.